Electrolytic image formation



DN 24, 1968 I c. o. ucMAsreR 3,418,217

' mcnomnc IIAGE FonuA'rIoN Filed July 23,1959

rus/rumana il United States Patent O 3,418,217 ELECTROLYTIC IMAGE FORMATION Clarence 0. McMaster, St. Paul, Minn., assigner to Minnesota Mining and Manufacturing Company, St. Paul, Minn., a corporation of Delaware Filed July 23, 1959, Ser. No. 829,124 13 Claims. (Cl. 2014-18) ABSTRACT OF THE DISCLOSURE This application discloses image forming processes in which an insoluble metal hydroxide precipitate is electrolytically provided on the light exposed areas of a photoconductive copysheet. The resulting metal hydroxide precipitate may be utilized in several ways, such as in lithography, or may be visually developed to provide a print of the original light image, such as by depositing silver or a dyestui in the alkaline precipitate areas of the copysheet.

This invention relates to the formation of permanent reproductions of light images on light-sensitive surfaces by methods involving electrolysis at the exposed lightsensitive surfaces. In one aspect, this invention relates to the formation of a latent or non-visible image capable of being visibly developed by non-electrolytic means. In another aspect, this invention relates to the formation of a precipitate image on a light-sensitive surface by electrolysis and to the development of such precipitate images.

The use of photosensitive sheet materials having surface layers which become electrically conductive when irradiated with light of certain Wave lengths is known. Thus, a photo-conductive material such as zinc oxide can be coated on a sheet of electrically conductive material, exposed to a light image, and subjected to electrolysis in the presence of various reducible developer solutions, such as an aqueous silver nitrate solution. The differential conductivity pattern produced lby illumination with the light-image is thereby electrolytically developed by reduction of the developer material to form a visible reproduction of the original light image. Both positive and negative reproductions can be made in this manner.

Although the above process is capable of producing permanent visible reproductions having good detail and contrast, the diiferential conductivity pattern produced on the photo-conductive layer has a limited life. Photoconductive zinc oxide layers, for example, have a measurable half-iife of decay which is under one minute, usually about 30 seconds or less. Because of the relatively transicnt nature of the photoconductive image, it has been necessary to develop the nal visible image either simultaneously with or shortly after the original exposure. Frequently, it is desirable to elfect the actual development of the visible image at a later time, particularly when making a series of exposures to lbe developed by dilferent developing methods. The necessity for interchanging various developer solutions and for maintaining an available inventory of the various developer solutions has been inconvenient, particularly when such reproductions are made only infrequently.

As mentioned earlier, although the use of an electrolytically reducible developer solution, such as aqueous silver nitrate solution, produces images with good detail and contrast, it has also `been found that areas of photoconductive material subjected to light of a given intensity tend to have points of slightly different conductivity and that developer, e.g. reduced metal, in those areas is there- 3,418,217 Patented Dec. 24, 1968 ICC fore not entirely uniformly deposited. Microscopic examination of such electrolytically developed areas reveals the somewhat grainy nature of the electrolytically deposited developer material. It is additionally often advantageous to eiiect development of the visible image Without reduction of the developer material.

It is therefore an object of this invention to provide a novel method for reproducing images on exposed photoconductive surface,

It is a further object of this invention to provide an electrolytic method for producing a permanent, relatively non-visible image on an exposed photoconductive surface.

Another object of this invention is to provide anelectrolytic method for producing a non-visible image on an exposed photoconductive surface which can be maintained for an indefinite period of time before development of the visible image.

A further object of this invention is to provide a permanent image on an exposed photoconductive surface which can be stored indefinitely and subsequently ybe further visibly developed by non-electrolytic means.

Still another object of this invention is to provide a new method for reproducing both non-visible and visible images on an exposed photoconductive surface which can be developed by non-electrolytic means.

It is a further object of this invention to provide an electrolytic image on an exposed photoconductive surface which has improved grain characteristics and exceptionally good contrast.

Yet another object of this invention is to provide a novel photoconductive sheet capable of use in accomplishing the above mentioned objects.

In accordance with this invention, the above and other objects are realized by activating with a light image a receptor sheet having a strongly photoconductive water resistant layer on a contiguous electrically conductive backing and electrolyzing a solution of` a conducting metal salt, the metal of which forms an insoluble hydroxide in alkaline media, using the electrically conductive backing as the cathode. The electrolytic current flow pattern through the photoconductive layer corresponds to the differential light pattern of the original image, the current How being greatest in those areas which received the most intense irradiation or light exposure. Adjacent to the interface :between the light struck photoconductive surface areas and electrolytic solution, an area of relatively high pH is produced, the pH varying in direct relationship with the current oW. The metal ions in the electrolyte solution migrate to the cathode and then distribute diierentially over the surface of the photoconductive layer in accordance with the conductivity pattern, vwhere they react `with the excess hydroxyl ions and are precipitated as metal hydroxides which are insoluble in neutral and alkaline media. Simultaneously hydrogen gas is liberated at the cathode. The precipitate image formed in the above described electrolysis is permanent and stable to the effects of heat and light. It is unaffected -by alkali treatment and can withstand moderate `washing with water. Preferably, the precipitate image bearing surface is washed with water to remove residual water soluble salts before Storage or before subsequent development, since certain salts may tend to discolor upon aging.

As mentioned earlier, the electrolyte solution must contain a conducting metal salt, the metal ion of which is capable of forming an insoluble metal hydroxide in neutral to alkaline media. It is therefore an essential feature of this invention to select the metal of the conducting salt from those metals which produce insoluble hydroxides and further which are not reduced to a lower valence state during electrolysis and are therefore unsatisfactory in the instant process. Illustrative of the metal salts operative for the above purposes are the conducting salts of beryllium, magnesium, eg. magnesium chloride, and aluminum, e.g. aluminum nitrate. Magnesium salts are particularly preferred, since they possess the requisite alkaline properties and are relatively insoluble in neutral as well as in alkaline aqueous media. Generally those metal ions which are not reduced under mild electrolytic conditions, i.e. about 100 volts D.C. or less and current densities :below about 0.6 amp per square inch, are metals above uranium in the electromotive series of elements (Handbook of Chemistry & Physics, 30th ed., Chemical Rubber Publishing Co., page 1439).

The electrolyte solution may be introduced in a variety of ways. Hence, the light exposed photoconductive receptor sheet can be immersed in an electrolyte bath, with the electrically conductive backing of the receptor sheet being insulated or shielded from electrical Contact with the electrolyte. The electrolyte salts may also be included in or on the surface of the photoconductive layer, where the selective precipitation can be effected by merely wetting the surface with water or other suitable ionizing7 media during the electrolysis, e.g. by passing a wet sponge, connected as the anode, over the surface. As another example, the electrolyte may be contained in a gelatin or equivalent layer superimposed on the surface of the photoconductive layer. Such gelatin layers are relatively transparent and allow transmission of the light image to the surface of the photoconductive layer with only a minor loss in intensity. When such a gelatin layer is used, the precipitate image can be formed by slowly passing a heated metal rod, connected to serve as anode in the electrolysis reaction, over the surface of the gelatin.

During electrolysis the insoluble metal hydroxide is precipitated on the light struck areas of the photoconductive surface. When the photoconductive surface comprises zinc' oxide and the electrolyte comprises an ionizable magnesium salt, the precipitated magnesium hydroxide is invisible or only slightly visible on the zinc oxide background. However, if a colored photoconductive material, such as selenium or cuprous oxide, is employed as the photoconductive layer, the light-colored magnesium hydroxide precipitate is contrasted against the darker colored background areas, producing a visible reproduction of the original light image.

It will be understood from the above description that the above described precipitate image may be either visible or invisible. Where the precipitate image is immediately visible upon electrolysis the use of additional developer materials or developing steps may be obviated.

The accompanying drawing illustrates an exposed photoconductive sheet containing electrolytically deposited metal hydroxide precipitate on the light-struck areas of the photoconductive surface.

When necessary or desired, further development of the precipitate image is accomplished by taking advantage of the alkaline nature of the precipitate. Thus, a vivid, high contrast visual image may be produced without using a reducible developer and without further electrolysis by bringing the precipitate bearing surface into contact with a material which changes in color value at a pH of above about 7, preferably above a pH of about 9. For example, any of the indicator dyes, such as malachite green, methyl red, thymol blue, alizarin red S, etc., may be employed for this purpose. Usually these indicators or developers a're applied from a solution or suspension onto the surface bearing the precipitate image by such conventional techniques as brushing, sponging, dipping, roll coating, spraying, etc. A color change is apparent in those areas containing the precipitated hydroxide, providing a visible reproduction of the original light image. Various indicators that undergo a color change in alkaline media can be used, selection being primarily guided by the desired color-contrast and the desired degree of permanence.

Besides the indicators mentioned above, it is also within the scope of this invention to effect final development of the visible image by treating the image bearing surface with a metal ion that forms a colored precipitate in alkaline media. A dilute, aqueous solution of a silver salt, such as silver nitrate, etc. is particularly preferred, since its application to the image bearing surface produces an immediate, intense, high contrast brown-to-black deposit on the alkaline precipitate areas. A negative image with excellent definition is thus formed. The grain structure and uniformity of the deposit is markedly improved as compared to a similar metal deposit obtained by electrolytic reduction of the soluble silver salt at the photoconductive surface. Development results from the precipitation of silver, apparently in oxide or hydroxide form, on those surface areas containing the water insoluble metal hydroxide precipitate; however, for purposes of this invention, this precipitation will be referred to as the precipitated silver compound.

Where an improvement in light stability of the developed image is desired, the surface may be fixed by washing with water to remove the excess silver salt or by complexing the silver to prevent subsequent discoloration.

When using a silver salt as the indicator or developer it is preferred embodiment of this invention to improve the quality of the reproduction by developing with a complexed silver salt, such as an aqueous solution of a silver-thiourea complex. Excellent stability to high humidity and light is obtained by developing with a three-to-one molar ratio of thiourea to silver nitrate.

It should be recognized that the presence of the precipitate image permits a variety of methods for subsequent visible development, based on the differential surface alkalinity provided by the insoluble metal hydroxide. However, the precipitate image can be further developed by other methods which utilize the -natural electrical resistance of the precipitate. The precipitate image bearing surface can, for example, be flooded with light and subsequently electrolyzed in the presence of 'a reducible electrolytic developer, such as silver nitrate. This produces a dark silver back-ground for the precipitate image. If, for example, the precipitate image is formed of magnesium hydroxide and copper or nickel is electrolytically reduced and plated out on the background areas after flooding with visible light, a positive reproduction is obtained. However, either negative or positive images can be produced.

The receptor sheet which is exposed to the light image and on which the precipitate image is formed contains an electrically conductive base, such yas metal foil, upon which a photoconductive layer of a material having a photoconductivity value of at least about 10-rl mho./cm. and a dark conductivity value not ygreater than about onetwentieth of the photoconductivity value, is placed or bonded. In bonding the photoconductive material to the electrically conductive base various water resistant, exible adherent lm forming polymers can be used, provided the polymer is light in color and does not adversely atect the light sensitivity of the photoconductive material. Such polymers include a 30:70 copolymer of butadiene and styrene (Pliolite S-7 solution, 30% solution in toluene), polystyrene, chlorinated rubber, rubber hydrochloride, etc. Polymers which are dissolved or softened by water, or which are dark in color, or insoluble in commercial solvents, or reactive with the photoconductive material, or which readily wet the photoconductive particles, are found to be less effective. Thus, polyvinyl alcohol, polyacrylic acid, shellac and sodium carboxymethyl cellulose are generally not used as binders for the light-sensitive materials.

As light-sensitive materials, zinc oxide, cadmium sulde, and other photoconductive powders having equivalent apparent photoconductivity values, as above described, have also been found to provide ladequate photoconductivity values in coated film form and to produce receptor sheets suitable for use in the instant invention. Mixtures of the photoconductive materials may also be employed.

To improve or enhance the sensitivity of these photoconductive materials in certain visible and non-visible areas of the light spectrum, dye sensitizers, such as acridine orange, i'luorescein, eosin Y, rose bengal, methylene blue, etc., are preferably admixed with the photoconductive powder.

Metal foil or sheet provides a suitable electrolytically conductive backing. Metal conductors, such as aluminum, chromium, nickel and copper, are suitable for the electrically conductive backing and may additionally be placed on the surface of a non-conductive supporting sheet, e.g. by vapor deposition, lamination, etc.

In one illustrative example, a coating composition is prepared by first mixing together 640 grams of photoconductive zinc oxide pigment (New Jersey Zinc Co. USP-12), 533 grams of a 30% solution of a 30:70 copolymer of butadiene and styrene in toluene (Pliolite S-7 solution), and 353 grams of acetone. The mixture is then milled for about 8 hours in a one-gallon ball mill loaded to about half its volume with l1/2 inch diameter porcelain balls, The resulting slurry or suspension is thick and viscous :but ows readily and can be spread with a coating knife to form a smooth uniform coating. This suspension is coated on the clean metal surface of an 81/2 x 11 laminate of thin paper and thin aluminum foil, and the solvent removed by evaporation, to provide a smooth uniform dried coating about 0.8 mil thick. The resulting sheet is exible and the coating remains firmly bonded to the metal during handling or rolling of the sheet.

The above sheet, previously held under dark conditions for at least one-half hour, was exposed to a light image for about 5 seconds. The negative pole Of a 50 volt D.C. source of potential was then connected to the metal foil of the sheet, as by means of edge clamps, the positive pole being connected to a narrow strip of fine-grained cellulosic sponge partly saturated with a 2 wt. percent solution of magnesium chloride. The sheet was drawn past the sponge at a constant rate such that each point of the surface remains in contact with the sponge for about 0.4 second. The sheet remained substantially dry and precipitated magnesium hydroxide was deposited at the light-struck areas.

The sheet containing the precipitate image was thereafter developed by immersion in a 1 Wt. percent solution of silver nitrate containing thiourea in a silver/thiourea mole ratio of l/3. An immediate formation of a dense brown-black image was noted, which image was unalected :by light or humidity. Microscopic examination of the dark areas revealed a relatively uniform grain structure.

It is additionally within the scope of this invention to use the photoconductive sheet containing a precipitate image as a simple, inexpensive and effective lithographic sheet for .use in conventional lithographic processes, e.g. Multilith. Since the precipitate image generally displays hydrophilic properties in relation to the background or non-light struck areas, the surface of the photoconductive sheet displays ink receptive properties in the relatively hydrophobic background areas and ink rejecting properties in the relatively hydrophilic precipitate image areas. This selectively coated photosensitive sheet having varying degrees of ink receptivity, corresponding to the original light image, can be employed successfully in lithographic processes, particularly when a rapid and economical limited run is desired.

When similarly exposed photoconductive sheet containing a precipitate image was developed `by immersion in dilute aqueous solutions of malachite green, methyl red, thymol blue and alizarin red S, vividly colored reproductions Aof the original light image were obtained.

Various alterations and modifications of the present invention will be apparent to those skilled in the art without departing from the scope of this invention.

I claim:

1. A process for the reproduction of a light image which comprises exposing to said light image a strongly photoconductive coating on a contiguous electrically conductive backing, said photoconductive coating containing a photo conductive material having a photoconductivity value of at least about lO-7 rnho./cm. and a dark conductivity value not greater than about one-twentieth of the photoconductivity value; electrolyzing an aqueous solution of an ionizable metal salt, the metal of which forms an insoluble hydroxide precipitate in alkaline media and is above uranium in the electromotive series, in contact with the light exposed surface area of said coating, said electrically conductive `backing being connected as the cathode; and subsequently contacting said precipitateimage with a developer which selectively and visually changes color in an alkaline environment.

2. The process of claim 1 in which the developer is an indicator dye.

3. The process of claim 1 in which the developer is a solution containing a metal ion which forms an insoluble colored precipitate in alkaline media.

4. The process of claim 1 in which the developer is a dilute solution of a Silver salt.

5. The process of claim 1 in which the developer is a dilute solution Aof silver nitrate and thiourea, the mole ratio of silver nitrate to thiourea being about 1:3.

6. A sheet containing a reproduction of a light image which comprises a strongly photoconductive coating on a contiguous electrically conductive backing, said photoconductive coating containing a photoconductive material having photoconductivity value at least about IGH mho./ crn. and a dark conductivity value not greater than about one-twentieth of the photoconductivity value7 said photoconductive coating containing on the light exposed areas a magnesium hydroxide precipitate and, superimposed thereon, a precipitated silver compound.

7. A sheet containing a reproduction of a light image which comprises a strongly photoconductive coating on a contiguous electrically conductive backing, said photoconductive coating containing a photoconductive material having a photoconductivity Value at least about l0"7 mho./cm. and a dark conductivity value not greater than about one-twentieth of the photoconductivity value, said photoconductive coating containing on the light exposed areas an aluminum hydroxide precipitate and, superimposed thereon, a precipitated silver compound.

8. In a photoconductolithographic process the steps comprising electrolytically depositing on a hydrophobic photoconductive layer a hydrophilic image consisting essentially of magnesium hydroxide and lithoprinting from the layer and hydrophilic image.

9. The process according to claim 8 in which the photoconductive layer is zinc oxide in resin binder.

10. A process for the reproduction of a light image which comprises exposing to said light image a strongly photoconductive coating on a contiguous electrically conductive backing and electrolyzing an aqueous solution of an ionizable aluminum salt in contact with the surface areas of said coating corresponding to the light exposed areas, said electrically conductive backing being connected as the cathode.

11. A light exposed sheet which comprises a strongly photoconductive coating on a contiguous electrically conductive backing, said photoconductive coating containing on the light exposed areas an insoluble aluminum hydroxide precipitate.

12. A sheet for the reproduction of a light image which comprises a strongly photoconductive coating on a vcontiguous electrically conductive backing, said photoconductive coating containing on its surface an ionizable aluminum salt.

13. A sheet for the reproduction of a light image which comprises a strongly photoconductive coating on a contiguous electrically conductive backing, said photoconductive coating having on its surface a gelatin film containing an ionizable metal salt, the metal of which forms an insoluble hydroxide precipitate in alkaline media and is 168,465 10/1875 Edison 96-1 above uranium in the electromotive series, 168,466 19/1875 Edison 96-1 References Cited FOREIGN PATENTS 5 215,754 6/ 1958 Australia. UNITED STATES PATENTS 1511971 5/1904 Germany* 10/1962 Sagura 204-18 9/1962 Reithel 204--15 JOHN H. MACK, Primary Examiner. 11/1961 Johnson et aL 204-18 T. TUFARIELLO, ASSI-mmf Examiner. 11/1896 Langhans 20456 10 12/1942. Solomon 204- 2 US. C1. X-R.

3/1875 Edison 96-1 96-1 

